The invention concerns improvements to chambers in which a bed of solid particles flows, termed a moving bed. It is particularly applicable to moving bed reactors, in particular to moving bed catalytic reactors such as those used for catalytic reforming.
In moving bed units in which the solid, which may or may hot be catalytic, and may or may not be spherical, is circulated in a dense bed which flows vertically under gravity, the walls which delimit the granular zone usually comprise 2 cylindrical screens of differing diameters. The fluid, which may be liquid but is usually gaseous, or possibly the fluids (gas and liquid, or liquid and another non-miscible liquid) traverse the granular bed (annular in this case) as a cross-flow, i.e., a radial flow, from outside to the inside, or conversely from inside to outside. After passing through the cylindrical inlet screen, the fluid then traverses the bed, then leaves the annular space containing the granular medium by passing through a second screen, the outlet screen, which is concentric with the inlet screen.
The flow of the fluid(s) traversing the bed causes a pressure drop which depends on a number of factors, the major factors being the size and shape of the particles, the properties of the fluid or fluids and the flow rate of the fluid(s). This pressure drop causes a thrust by the fluid on the solid particles in the direction of flow of the fluid. This thrust is directed towards the outlet screen and modifies the balance of forces to which the solid particles are subjected (spherules, for example). This phenomenon causes problems, such as slowing the movement of the particles which are against the wall, or even causing them to jam against the outlet screen and completely stop, which both severely inhibits the function of these particles and can cause other particles to slow or stop and gradually completely block the bed (for example a catalytic bed) and completely halt the circulation of the solid. This phenomenon is all the more accentuated when the thrust on the fluid(s) is high and thus when the capacity of the unit is high.
Blocking thus constitutes an important limitation on increasing the capacity of the units. The development of arrangements and means to reduce the size of this phenomenon is thus of particular interest.
This problem is acute in catalytic units such as catalytic reforming units. The risk of the catalytic particles jamming against the screens severely reduces the flow rates of the gas to be treated and thus severely reduces the capacities of the units.
Further, during different treatments, in particular regeneration of the catalytic particles, or transfers between reactors or between reactors and regenerators, spherule fragments are formed which rapidly jam in the screens. This fragmented population thus accelerates blocking.
The presence and irregularity in the concentration of broken or small particles (substantially less than the average size of the particles) in some zones of an annular moving bed can cause a number of anomalies.
If there is a greater abundance of fine or broken particles in some zones, they locally reduce the porosity of the bed which limits the velocity of the fluid and thus prolongs the contact time of the fluid with the solid particles. In contrast, in zones which are depleted in fine particles, the porosity is higher and the fluid passes more easily and thus faster and has a smaller contact time with the solid. This heterogeneity in the contact time can cause a number of problems, in particular:
a) For the reaction fluid (in particular a gas): PA1 b) For the particles: PA1 irregularities in the fluid flow can cause substantial differences in the amplitude of the various deactivation phenomena caused by contact with the reactants, in particular poisoning or coking. This enhanced deactivation further reduces the efficiency of the catalyst in some zones and can then cause a reduction in the expected degree of conversion and the degree of selectivity; PA1 an increase in the residence time of some particles of the bed also causes variations in deactivation of the catalytic particles, which reduces the overall performance of the unit. PA1 at least one screen delimiting a moving bed of solid granular particles of thickness e, which may or may not be constant, which circulates in the chamber in a downward direction substantially parallel to the screen; PA1 at least one opening for the introduction of at least one fluid which circulates in said bed in a principal direction which is different to that of the ensemble of particles and leaves the bed via at least one outlet screen; PA1 at least one means for extracting the particles from the chamber; PA1 at least one opening for the introduction of particles into the chamber, via at least one means for distributing said particles, said means being located above the moving bed of particles and said means comprising at least one orifice for the stream of particles, each orifice(s) being located at a distance of at least 0.6 e from the outlet screen. PA1 by a screen and a wall. This is the case, for example, when the screen is cylindrical and the bed circulates between the cylindrical wall of the reactor and the screen; PA1 by at least two screens, an inner screen and an outer screen. Preferably, the screens are parallel in pairs or concentric. This is the case, for example, with two cylindrical screens located concentrically along the reactor axis, the bed circulating between the screens.
overall degrees of conversion which are lower than the values obtained in the presence of a regular, well distributed fluid flow; PA2 local lack of temperature equilibrium when the heat of reaction is not zero, and as a consequence; PA2 local variations in the nature and quantity of the products obtained, in particular secondary products, thus limiting the overall selectivity which is sought; PA2 local irregularities in the rate of flow of the fluids and local differences in the size distribution, and even the shape of the particles, modifying the flow characteristics of the bed, such that the downward movement of the various zones of the bed can become irregular, with the following principal consequences:
particles which can move faster to the fluid outlet screen encrust themselves in the orifices of the screens and hence perturb sliding of the granular bed against the walls or block some zones which has a particularly deleterious effect on the performance of a moving bed unit; PA3 an increase in pressure drops through the fluid outlet screen.
There are further complications for catalytic particles, since: